Selective / Differential / Enrichment media
Selective media helps select for growth of certain organisms in a mixed population by using a ingredient that inhibits the growth of other microorganisms, but not the desired species or group. Enrichment media selects for certain microorganisms by including a nutrient that the desired microorganism or group can use and its competitors can not. (Sometimes enrichment media also limits alternate sources of nutrition). Differential media does not select for any particular group by inhibiting or enhancing their growth over competitors, but it does show a visible difference between or among groups of microorganisms. Some media can be both selective and differential.
For more information on the formulations and types of media available in microbiology see: BD diagnostice Systems Difco catalog of mediahttp://www.bd.com/ds/technicalCenter/inserts/difcoBblManual.asp

Selective for Gram negative Organisms

Eosin–Methylene Blue (EMB) Agar is a differential medium for the detection of Gram negative enteric bacteria. The medium contains peptone, lactose, sucrose, dipotassium phosphate, eosin and methylene blue dyes. Eosin and methylene blue act as indicators to differentiate between Gram negative organisms that ferment lactose and those that do not ferment lactose. Most bacteria that ferment lactose form colonies on EMB agar that are dark blue to black with a metallic sheen due to precipitation of the dyes by the acid by-products of fermentation. Colonies produced by lactose non-fermentors are not dark blue or black. The growth of Gram positive bacteria is generally inhibited on EMB agar because of the toxicity of methlyene blue dye. In low concentration, the protective lipid outer membrane of Gram negative bacteria prevents entry of the toxic water soluble dye while the more porous cell wall of Gram positive bacteria without the protective outer membrane makes them more sensitive to the toxicity of methyene blue.